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arXiv · 1809.07146

Non-thermal nature of photo-induced insulator-to-metal transition in NbO$_2$

Rakesh Rana·J. Michael Klopf·Jörg Grenzer·Harald Schneider·Manfred Helm·Alexej Pashkin
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Abstract

We study the photo-induced metallization process in niobium dioxide NbO$_2$. This compound undergoes the thermal insulator-to-metal transition at the remarkably high temperature of 1080 K. Our optical pump - terahertz probe measurements reveal the ultrafast switching of the film on a sub-picosecond timescale and the formation of a metastable metallic phase when the incident pump fluence exceeds the threshold of 10 mJ/cm$^2$. Remarkably, this threshold value corresponds to the deposited energy which is capable of heating NbO$_2$ only up to 790 K, thus, evidencing the non-thermal character of the photo-induced insulator-to-metal transition. We also observe an enhanced formation of the metallic phase above the second threshold of 17.5 mJ/cm$^2$ which corresponds to the onset of the thermal switching. The transient optical conductivity in the metastable phase can be modeled using the Drude-Smith model confirming its metallic character. The present observation of non-thermal transition in NbO$_2$ can serve as an important test bed for understanding photo-induced phenomena in strongly correlated oxides.

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Rakesh Rana, J. Michael Klopf, Jörg Grenzer, Harald Schneider, Manfred Helm, Alexej Pashkin. 2018-09-20. Non-thermal nature of photo-induced insulator-to-metal transition in NbO$_2$. https://doi.org/10.1103/physrevb.99.041102

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